The spectre of doping will once again hang over the Tour de France when this year's race begins on Saturday.

Pre-race favourites Jan Ullrich and Ivan Basso were both withdrawn by their teams on Friday after they were implicated in a doping investigation.

Tour boss Jean-Marie Leblanc said: "I hope we can clean up everything now. All the cheats should be kicked out.

"Then maybe we will get an open Tour with clean riders, with space for ethics, sport and entertainment."

Germany's Ullrich, the 1997 champion and third last year, Italy's Basso, second in 2005 and winner of this year's Tour of Italy, and Spain's Francisco Mancebo - fourth in last year's Tour - were excluded after their names were linked to an on-going blood-doping investigation in Madrid.

If they are eventually proven guilty, then cycling is better off without them

Pat McQuaid, president of the International Cycling Union

The Astana-Wurth team have also withdrawn en masse after five riders were named in the Spanish probe.

Although leader Alexander Vinokourov, fifth last year, was not one of them, he too will be absent with the team having too few riders left.

With last year's winner Lance Armstrong now retired, it means this year's Tour will start without the top five riders from 2005, with 13 in all evicted from the race.

The six-month Spanish investigation, nicknamed Operation Puerta, led to the arrests of five men, including Manolo Saiz, who recently resigned as boss of the Liberty-Seguros team, now re-named Astana-Wurth.

Tour organiser ASO (Amaury Sport Organisation) has revealed it has been given a list of more than 50 riders named in the investigation.

It is the biggest doping controversy to hit the race since the Festina affair in 1998.

None of the riders excluded from the race has been proved to have done anything wrong yet, but all the leading teams signed an accord on 1 January 2005 which stated they would not allow a rider to compete while they were under investigation in any doping affair.

Pat McQuaid, president of the International Cycling Union, said: "I'm sad that some of our top riders find themselves implicated in a doping affair but, on the other hand, if they are eventually proven guilty, then cycling is better off without them - we must insist on a clean sport."

Valverde is now one of the favourites

In the absence of Basso and Ullrich, the main yellow jersey contenders are likely to be Spaniard Alejandro Valverde, fifth overall when he was forced to retire last year, and Americans Floyd Landis and Levi Leipheimer.

Lithuanian Yaroslav Popovych and Australian Cadel Evans could also come into the equation.

"I hope we have a clean race, not only in the Tour but in all the other races in the calendar," said Valverde.

"I sincerely hope that whoever wins this Tour will be remembered as a great champion and not as someone who won a race stained by a doping scandal."

Talk of doping will inevitably dominate day one of the prologue, with Britain David Millar among the favourites to win the 7.1km stage in Strasbourg in his first race since serving a two-year doping ban.